Traffic of a vehicle-mounted network has increased due to the advancement of a safe driving support or an automatic driving technology. To cope with these problems, a CAN with flexible data rate (CAN FD) communication system which can increase a data transmission rate and extend a data length has been known. In the CAN FD, nodes which perform transmission and reception are generally electronic control units (ECUs), and each node is electrically connected by a bus. A transmitting ECU adds an identifier (ID) to communication data to construct a message, converts the message into an electric signal, and transmits the electric signal on the bus. Each ECU monitors the electric signal on the bus, acquires the ID during the communication, and specifies the message to be received.
When a plurality of messages are transmitted at the same time, priority of communication is determined according to the ID. A phase of determining the ECU which can transmit the ID and transmit the message according to the priority is called an arbitration (adjustment) phase. In the arbitration phase, the plurality of ECUs perform communication at the same rate as the conventional CAN, for example, at 500 kbps for simultaneous output. After the ECU which transmits the message is determined by the arbitration, the arbitration phase becomes a data phase for transmitting data. In the data phase in which the number of ECUs outputting the message is specified to be one, the transmission rate is 2 Mbps, for example.
However, there is a problem in that if a communication rate is increased to 2 Mbps in the conventional network configuration which can communicate at 500 kbps, data are not correctly transmitted due to reflection. As a method to solve these problems, it is effective to divide buses and reduce a scale of a network of each bus (reduce the number of connected nodes and reduce a length of harness (wiring)). However, in this case, there arises a problem in that the number of ECUs that can perform communication at the same time is reduced. In such a case, a method of using a gateway ECU to connect divided buses has been known (for example, see PTL 1). The gateway ECU transmits data received from one bus to the other bus. It is possible to perform the communication between the ECUs connected to buses divided into two by using the technique disclosed in PTL 1.